The 124 is actively balanced and buffered in both directions, with very low output impedances. A more expensive type of balanced input circuit is used for even higher hum and noise rejection. In addition, all inputs and outputs are carefully filtered for freedom from RF interference.

Some other devices are passive in one direction, simply using resistors to lower level from +4 to -10. The passive circuitry unbalances the input, eliminates all common-mode rejection allowing for hum and noise pickup, and presents a rather high impedance at the -10 output, permitting even more noise to enter the system.

Although more complex and costly, servo-balancing has many advantages over conventional balanced circuits. On the inputs, it means that high common mode voltages, such as induced hum from a nearby AC line, are compensated for, preserving headroom that is sacrificed in other devices (balanced input overload is greater than +30dBm with common mode input voltage tolerance of over 200 volts rms). The input impedance of 100K Ohms allows bridging over 600 Ohm balanced or unbalanced line with negligible loading effects. Front panel switchable 600 Ohm load allows for input line termination when required.

On the outputs, servo-balancing means that the outputs may be used balanced or unbalanced (single-ended) at any time by simply grounding either unused pin. The servo compensates for the voltage loss on one side so there is no 6dB loss of level as with other systems. The outputs are also short-circuit proof, so nothing is damaged if the wrong pins are grounded. Output impedance is very low so long lines are driven effortlessly. A front panel indicator will flash red when the balanced output level is 5dB below clipping.